This area is accessible in the "Message" section:

 

Sender (e-mail): This is the sender's e-mail address that your recipient will see when receiving the message.

Sender's name: Would you prefer to see your company's name as a sender rather than your e-mail address? Fill in this field.

Message subject: Indicates the subject, the purpose of your message ("New product", "Newsletter N°4" etc...)

 

Note that several additional parameters such as the reply e-mail address are available in the "Advanced settings" section of the message: